Psoriasis is a non-contagious chronic skin disease, the cause of which is still unknown and which is typically characterized by thick red zones of the skin which are covered with silvery or whitish scales. The extent of the disease is variable, with small local areas or the whole of the surface of the body being affected. The disease may also affect the joints, nails and the mucous membranes.
Topical therapies are usually used for the less severe forms of psoriasis which are generally well defined and have the classic symptoms. Creams and ointments with a pharmacological action (keratolytic agents, tar, dithranol, cortisone preparations, calcipotriol, tacalcitol, tazarotene) or simply softening agents are used.
Systemic therapies are used for the more severe forms or the forms of common psoriasis which do not respond to topical treatment. These are more complete and effective therapies since they are applied to the whole of the organism. Precisely for this reason, however, they may also cause notable side effects: it is essential, therefore, that they should always be administered under the close supervision of a specialist doctor.
The most common systemic therapies use: methotrexate, cyclosporin A and retinoids. All these treatments give rise to significant side'effects and contraindications which are well known in literature and to specialists in the sector.
Phototherapy is also widely used.
Exposure to the sun has always been used for the treatment of psoriasis; it is therefore logical to imitate this using artificial sources so that it is readily available. Phototherapy is based on the application of ultraviolet radiation (UV). In the same way as visible light, which can be divided into the spectral colors ranging from violet to red, ultraviolet radiation can be divided into individual bands of varying wavelength. This division is performed on the basis of the biological effect caused and thus the following three bands are obtained:
UV-C 100–280 nm: sterilization
UV-B 280–315 nm: skin erythema
UV-A 315–380 nm: skin pigmentation
The corneal layer absorbs most of the UV-B radiation which is then totally absorbed by the malpighian layer and by the dermis. The melanin absorbs, in addition to the heat, essentially the visible radiation; only a part of the UV-A radiation and the visible radiation reaches the dermis, while the red and infrared light passes through the epidermis and dermis and reaches the hypodermis.
There was a notable increase in the use of light sources for the treatment of psoriasis with the appearance of photochemotherapy (PUVA) which also awakened interest in other phototherapeutic methods. After a few years, however, the initial optimism regarding PUVA therapies was replaced by a certain skepticism; all the undesirable effects of the photosensitizing drugs became evident and the long-term side effects of the drugs used were not clear.
Over the years it was demonstrated that all the therapeutically effective radiant sources, including solar light (global radiation) use the range of 297 to 315 nm. In 1980 Parrish showed, in his experiments conducted on human beings, that the optimum efficiency in psoriasis treatment occurs in the range of 308 to 315 nm. This is the starting point of the principle of selective phototherapy, i.e. the use of a light source in which the spectral distribution of the energy contains a well-defined fraction of UV-B radiation (308–315 nm) necessary for obtaining the best therapeutic effect in the treatment of psoriasis.
At present two types of lamp are used: fluorescent lamps and metal—halogen lamps. The former have a wide active spectrum and must be used with filters which limit their spectrum, eliminating UVC and UVB radiation with a shorter wavelength; fluorescent tubes also have an emission with a wide spectrum and are limited to certain bands by specific coverings. Laser treatment of psoriasis has also been envisaged. This solution provides a very concentrated emission and therefore requires scanning along the surface to be treated. The use of laser and the implications of a concentrated emission mean that this solution is very costly and that particular care on the part of the operator is also required.
The solutions adopted hitherto also require the simultaneous use of drugs which may cause undesirable effects.
Moreover the use of coherent light, the wide nature of the range of radiation emitted and the use of corrective drugs are negative factors as regards specific treatment of the skin disorder in question.